Internet connections in the South East Asian nation have been affected by problems with the Asia America Gateway (AAG) submarine cable system for the fourth time in a year, according to local news outlets. The cause of the outages? Sharks, if you believe some online reports.

As AAG has the highest capacity of cable systems linking to Vietnam, faults can have big consequences for its international Internet bandwidth.

With 99 percent of all transoceanic Internet traffic now flowing through submarine cables, how large of a threat are these big fish? Michael Costin, Chairman of the AAG Cable Consortium, says the damage was likely caused by ship anchors or fishing.

"AAG is not yet aware of the cause of the advised fault, but strongly believes it is not caused by sharks," Costin said via email. "Consistent with past experience, AAG believes it is most likely to be as a consequence of damage due to ship anchors or fishing, which reportedly are the predominant causes of faults experienced by submarine cables."

The fault occurred 117 kilometers off Vung Tau in the southern part of the country. Costin said the consortium's local partner is working with Vietnam's coast guard to protect the AAG cable.

He points to research by the International Cable Protection Committee (ICPC), a global industry group which claims that 65 to 75 percent of cable faults are due to anchoring and fishing. It found no cable problems that were caused by sharks from 2008 to 2013, compared to 11 cases between 1959 and 2006 as documented by the United Nations Environment Programme's World Conservation Monitoring Centre.

The ICPC says advances in cable design have "effectively eliminated the problem." Modern cables wrap a core of optical fibers in multiple protective layers including a copper or aluminum tube as well as a fiberglass or plastic shell. Cables laid near shore or fishing grounds are often equipped with an additional shield of steel wires and buried.

"As for shark bites, its like a joke within this industry," Shota Masuda, a senior manager in NEC's Submarine Network Division, said via email. "Google made a remark last year about this, and this brought back a few papers written about this topic into the light, but in reality, sharks do not bite fiber optic submarine cables."

The continuing importance of marine cables has been highlighted by Google's recent investments in the infrastructure. NEC, one of the top three submarine cable makers along with Alcatel-Lucent and TE SubCom, is part of a consortium including Google working on the US$300 million FASTER cable system between Japan and the U.S. With six fiber pairs and 100 wavelengths, it will have a capacity of 60Tbps when it comes online in 2016. As Google described it, that's about ten million times faster than your cable modem.

NEC manufactured cables used in the 20,000km-long AAG system. Masuda says that the AAG Consortium is responsible for maintaining the link but said that cables can get cut by dropped anchors, trawler nets and undersea earthquakes. NEC's underwater cables, he said, are rated to last for 25 years at depths of up to 8 kilometers.

Biologists, meanwhile, say sharks are known to bite submarine cables. How much damage they can cause depends on the species and the cable itself.

"Underwater cables are sometimes bitten and damaged by sharks," Kazuhiro Nakaya, a marine scientist and professor emeritus at Hokkaido University, said via email. "Although 8,000m below the sea level is a little too deep for sharks (the deepest shark record is less than 5,000m), it is possible that the cables settled shallower areas are damaged by sharks."

Sharks are attracted by electromagnetic fields they can pick up through sensing organs called electroreceptors, which are located in their snouts and heads.

"They cruise along the bottom trying to detect bioelectric fields from prey living on the bottom and also looking for the light flashes of bioluminescent prey," Dean Grubbs, an associate scholar scientist at Florida State University Coastal and Marine Laboratory, said via email. He was referring to the bluntnose sixgill, which he said is the shark in the 2010 video on YouTube.

"Though their jaws are relatively weak, they have saw blade-shaped teeth and tend to bite large food items (including carrion like dead whales) and then twist their large body until they saw out a hunk of flesh. It is possible that a large shark feeding in this way could damage submarine cables."

Grubbs said he has seen experiments showing sharks orienting to a weak electromagnetic field emanating from an electrode. A transatlantic cable laid in the 1980s by AT&T was found to have been damaged several times by undersea attackers, primarily crocodile sharks, Grubbs said. He added, however, that "most deep-sea sharks have relatively weakly calcified jaws so their bite force is quite low."

That, combined with better protection for cables than in the past, suggests human activity is what has been affecting the Internet in Vietnam. According to the management consortium's Costin, anchoring and fishing were responsible for all the faults last year. It's expected to be fully restored by Jan. 27.

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